This invention relates generally to electro-optic elements constructed of twisted nematic liquid crystals and, more particularly, to optical liquid crystal elements, having improved contrast and visibility angle characteristics relative to electro-optic elements constructed of nematic liquid crystals with twisted arrangements
Electro-optic elements having a structure in which an electro-optic effect type liquid crystal panel is interposed between a pair of polarizing plates are known. For instance one such element is a twist-nematic mode (TN mode) using a nematic liquid crystal with molecular orientation twisted 90.degree., or having a super twist-nematic mode (STN mode) using nematic liquid crystal with molecular orientation twisted by an angle equal to or greater than 90.degree. but smaller than 360.degree..
Specifically, when such an electro-optic element is used as a direct-vision display device, visibility angle characteristics, such as contrast and color tones are inferior compared to CRTs widely used. This is considered a drawback of the liquid crystal display device.
FIG. 1 schematically shows the construction of a conventional TN-mode electro-optic element of a normally black display type in which having axes extended in the same direction polarizing plates to give a parallel-Nicols state and which shuts off light at the time of de-energization. This Figure also shows the relationship between the orientation of the liquid crystal panel and the directions of the transmission axes of the polarizing plates. Light from a light source 21 passes thorough a polarizing plate 24 having a transmission axis 23 at an angle of .OMEGA./2 to a reference line 22 normal to the transmission axis 23 of the display surface and changes into light linearly polarized in this direction and to be incident upon a TN cell 25. The inner surface of a lower substrate 26 of the TN cell is rubbed for orientation in a .OMEGA./2 direction indicated by the arrow 27 while the inner surface of an upper substrate 28 is rubbed for orientation in a direction 29 intersecting the former at an angle of .OMEGA.. The light introduced into the TN cell 25 travels therethrough while rotating the plane of polarization by an angle of .OMEGA., and thereafter emerges out of the TN cell. During de-energization of the TN cell 25, the illumination light is shut off by a polarizing plate 31 having a transmission axis 30 perpendicular to the plane of polarization of the emergent light. If a voltage is applied to the TN cell 25 so that the molecular orientation of the liquid crystal becomes perpendicular to the substrate surface, the above-mentioned rotation of the linearly polarized light is cancelled, thereby enabling transmission of the light through the polarizing. The value of this voltage is selected to change the quantity of light passing through the TN cell. FIGS. 2a to 2c show the display characteristics of this conventional construction The character of this construction is .OMEGA.=90.degree., and the product of the refractive index anisotropy .DELTA.n (about 0.093) and the thickness d (about 5 nm) of the TN cell is .DELTA.n.multidot.d=470 nm. FIG. 2a shows a visual angle dependency of the contrast in the horizontal direction of the TN cell of the above-described construction, FIG. 2b shows a visual angle dependency in the vertical direction, and FIG. 2c shows coordinates representing display color tones with voltage application and with not voltage application.
Optical liquid crystal elements of this conventional construction entail the following drawbacks. First, regarding the frontal characteristics, the black display in a normally black construction during the applied with voltage application is colored in the case of TN-mode or STN mode liquid crystal elements. This phenomenon has been found in several models. Ordinarily, if linearly polarized light is introduced into a TN-mode or STN-mode liquid crystal panel, elliptically polarized light emerges. If in this case the transmission axes of the polarizing plates and the orientation of the liquid crystal cell are determined the emergent light is turned into linearly polarized light in a certain direction with respect to almost all wavelengths, thereby avoiding the problem. However, light actually passing through the liquid crystal is visible light having a wavelength range of about 400 to 700 nm. In particular, in this wavelength range, blue light having a wavelength of about 450 nm and red light having a wavelength of about 610 nm emerge in elliptically polarized states. This is because the polarized state of the light varies with respect to the wavelength due to the influence of the optical rotatory dispersion in the liquid crystal, i.e., variations in optical rotatory power with respect to different wavelengths. Under this influence, a small amount of blue and red light comes through the TN cell when the voltage is withdrawn for the black display, and the display color is not black but purplish. Referring to the CIE color coordinates shown in FIG. 2c, a color coordinate point 37 greatly deviated from the center at the time the voltage is withdrawn. The arrow 38 on the coordinates indicates a change in the color tone as the voltage is applied to the TN cell. In the case of the STN-mode liquid crystal cell, the optical rotatory dispersion is large compared with the TN mode cell and the display blue or yellow. The problem of this coloring, due to the optical rotatory dispersion, is particularly considerable in the case of STN-mode liquid crystal panels having a large twist angle of 270.degree. which possess a large degree of coloring. To avoid this problem, a method of using a liquid crystal cell for optical rotatory dispersion compensation and a method of disposing at least two phase elements (Japanese Patent Unexamined Publication No.63-271415) has been proposed. However, the former entails problems such as an increase in the production cost, a decrease in the accuracy of the gap between the two cells and a reduction in the transmittance since it utilizes two liquid crystal cells. The latter does not sufficiently reduce the degree of coloring and reductions in the contrast and the transmittance also occur. On the other hand, recently, the industry has received demands for improvements in direct vision type full-color TVs utilizing TN liquid crystal cells and liquid crystal projection type TVs having specifically, improved image qualities. The problem of coloring due to optical rotatory dispersion is considerable in this field. For color display liquid crystal panels, a method has been proposed in which the thicknesses of the liquid crystal layers for R/G/B pixels are optimized with respect to the wavelengths of the respective colors to compensate for the optical rotatory dispersion (Japanese Patent Laid-Open No.60-202423). However, this method is difficult in terms of manufacture, i.e., in obtaining a difference in level and requires optimization of the optical system in its entirety, and is not suitable for achieving a desired contrast while avoiding coloring.
Second, these types of optical liquid crystal elements have a visibility angle dependency. That is, as the angle of visibility is increased, the contrast becomes lower and the color tone changes, resulting in the deterioration of the image. This is because the elliptically polarized components of the emergent light change with and increase in the angle of visibility, so that light comes through a liquid crystal display panel in the shut off state if the display is viewed obliquely, and because coloring due to the optic rotatory dispersion takes place together with this phenomenon to change the contrast and the color tone with the angle of visibility. Such contrast and color characteristics in arbitrary three-dimensional directions can be calculated and predicted by a numerical analysis based on, for example, Berreman 4.times.4 matrix method with a three-dimensional model of liquid crystal arrangements in the liquid crystal cell, and then the optical design of the liquid crystal panel can be determined on the basis of the results of this calculation. Thus, it is generally considered that the optical characteristics of the liquid crystal cell essentially relate to the visibility angle dependency. In the case of examples of the visibility angle dependency shown in FIGS. 2a and 2b, the contrast ratio is lower than 30 at an angle of 30.degree. both in the horizontal direction and in the vertical direction.